Fresnel Region Fields of Aperture Antennas Used in EMC Radiated Emission Testing

MIL-STD-461G RE102 specifics limits for radiated electric field emissions from military ground, ships, submarines, and aircraft. Additionally, MIL-STD-464D, defines the required electromagnetic environment for testing system venerability. Testing is conducted over the range of frequencies from 2 MHz to 18 GHz and typical antennas used to cover this range of frequencies include an active vertical dipole, a Bicone, and the double ridge guide horn. Pyramidal horn antennas used to generate a required electromagnetic environment (per MIL-STD-464D) are often positioned near the test system and consequently in the near field or Fresnel region of the radiator. Our group is undertaking a broad based experimental and numerical study to better understand the nature of the near field radiation characteristics of aperture antennas and this paper shall provide a summary of our accomplishments to date. Our results demonstrate that the near field gain of a smaller aperture antenna may be larger than that of a larger aperture antenna. This simple fact can be overlooked by practicing EMI test engineers who are accustomed to choosing a higher gain horn to generate a larger electric field.